Legally, we must avoid copyright violations; otherwise, we can be sued or even shut down. That's not debatable.

To satisfy academic requirements, it's also a good a idea to avoid plagiarism. Not all writers understand what the requirements are in this area. There is some overlap with respecting copyright, but there's more to it.

Perhaps most important of all, to merit the trust of our readers we need to provide references for just about every idea that is not common knowledge. This gives readers assurance that we didn't simply make up the information, that it's not just our own opinion. It has the added benefit of giving them an easy path to finding out more about the topic.